This has been happening recently but this has happened to me in 1.17 as well. Whenever I start up Minecraft, I notice that my GUI Scale is smaller. I didn't think much of it and went into the settings to change it and that's when I saw that the max was 0 and the least was -1. Usually, the range is -1 - 1 and I keep it at one. I found that the only way to retain it back is either redownloading Minecraft or restarting my phone. That's when it's fine but if I leave the app or restart the app, it resets back to 0. Sometimes when I reset my phone, the range changes to -2 - 0 with 0 being the equivalent of 1. I don't know why and I've searched here and elsewhere online and don't see a fix. I have screenshots of when I reset my phone and when I restart the app.