I’m new too, but I’ll try to help you somehow.
There are several factors that could be causing this behavior, specific settings or interactions, workflow actions, conditions. It would be important for you to check some settings - maybe you’ve already done this.
Are there workflows related to the Pop-up? Mainly of the “Hide” type.
“When an input is focused” → “Hide Pop-up”.
“When an input value is changed” → “Hide Pop-up”.
Also take a look at the “Conditional” tab of the Pop-up element, see if there is a “This element is visible” linked to some rules/conditions.
Maybe it would be interesting to do a test by starting something new.
Add a new Pop-up element to the page, include a new input and multilineinput element, reproduce the interactions and see if the behavior is the same or not.
Thank you for the feedback, it’s this positive energy that motivates me every day. I think you should be focused on your app right now, but if you want to test it and try it out in practice, DM me and I can help you!