Obviously bad practice, rude, inconsiderate, etc. Not normal on THS, but normal these days in general in most aspects of life. It’s been at least a decade since being considerate and communicative was something valued in most workplaces, so people learn and adapt.
You definitely dodged a bullet. The only thing you owe this HO is a thank you for showing her colors.
Early on we had a HO do some fairly similar things. She asked if we could schedule a video call, I promptly responded yes with some times we could do in the next few days. Ghosted me for days. Missed the first scheduled call. Hey, life happens right? Finally did a call, she said she loved us and I asked her to accept our application promptly. A couple days later still not done. I put in some other applications. Messaged the original HO saying we are looking at other options. Finally got accepted for another sit so I went in and withdrew our application. Almost immediately got a “hey, I thought you were going to sit for me message”. I only stuck with it because the house was beautiful in the exact town we wanted. But the sit we ended up with was so excellent we will be going back for a third time soon! Silver linings. 
We applaud this mindset. Way to go!
A significant portion of listings that we choose not to apply, or applications that we choose to withdraw, are due to perceived employer mindset of pet parent. Some people just can’t help themselves. Not for us, good luck to them.
Equally, we have a range of past pet parents with whom we developed a great initial fit and ultimately a strong relationship. We keep in touch. Often we return to sit again. Two-way respect for a fair exchange.
Like you, we try to set the tone early in the process.
Host bad practice… absolutely bad practice and, as a host, it irks me to hear about such bad communication. Having said that, that does happen both ways. A couple of times I have had a sitter that I am even attempting to choose, and wanting to confirm with them re questions, and I don’t hear back from them for days and then suddenly hear that they’ve accepted another sit.
If I’ve seen that a host has read my messages and not responded within a reasonable time (12 hours), I pull my interest and move on. It tells me that we’re not a good fit since I always respond within a reasonable time period, and I expect the host to as well.
That’s a surprisingly short time. It’s the shortest I’ve seen on this forum. But, of course, if it works for you, that’s what really matters.
That’s 12 hours AFTER I’ve seen that the host has read the message. If they can’t respond within that time, I really don’t want to waste my time waiting. There could be other opportunities I’d be passing up by waiting. Also, I think the courteous thing to do is to respond even if the person says, “I’ll get back to you soon”.
THS says for HO’s to respond within 72 hours. 12 hours is ridiculous. What about different time zones, traveling, no Wi-Fi, etc. You would be missing out on some great sits.
If they can’t at least acknowledge that they’ve received the message (as is evidenced by the “read” notice), it’s a red flag for me and I’m moving on. Have I extended the time for really good sits, “yes”. However I’ve also made a mental note that these are HO’s who might not respect the time and the fact that sitters are looking at and responding to other sits.
Everyone has to decide for themselves what works for them, of course. I guess I just personally prefer to extend a bit more lenience toward people & allow there could be many reasons why they might not be able to respond in that moment.
In point of fact, several of my best (& repeat) hosts started out with a long delay in responding, that ended up being something they couldn’t help. But because I was patient, gracious, & persistent, they were especially accommodating to make up for their delay — & I got the sit over others in what was a competitive location.
My thought is that there’s always time to cancel my first application if another equally great sit gets offered, so I really lose nothing by keeping it live unless & until that happens. 
I’m with you on this, I don’t why a host may take their time in replying and I’m happy to wait longer than 12 hours. I’ve had very few acknowledgments that my application has been received and read, much as I might like them they seem to be a rarity. In my application I make it clear I like a video call on WhatsApp to chat and that is invariably the next step.
No reply within 12 hours is definitely not a red flag to me.