Consider doing a factory reset. There's usually a menu choice that'll do it, I want to say within the Security settings. In lieu of that, on most Android devices it's some combination of pressing the Power and Volume buttons (I'm not familiar with ZTE anymore so you'll have to look it up on the net). Back up any files, pics and music that you'd like to save either to the cloud or your SD card if present. However you reset first power down and remove the SD card and then power back up to do the reset or the reset will wipe your SD card. A good factory reset can clean out most any hick-ups that your device is experiencing. It'll take some time for you to download your apps again and rebuild your device, but backing up everything can help speed things along. A good launcher like Nova Launcher or Action Launcher that allows you to back up your theme and layout can save you some time (I just had to rebuild my device myself).
Otherwise it could be the OS that your device is now using isn't compatible with said app(s). I was until this past weekend using the Moto G4 and a version of the Quickpik gallery app that someone had slipped me. The version of Quickpik - a well regarded gallery app - in the Play Store is now owned by Cheetah Mobile which is known for snooping. Someone was kind enough to slip me a clean version of the app which I side loaded and doesn't have Cheetah's snooping in/on it. My G4 and my side loaded version of Quickpik were playing very well with each other, however this past Sunday I banged up my G4 and because it was within warranty it was replaced by the merchant that I purchased it from with the new Moto G5 (Moto isn't making the G4 how I had mine tricked out anymore). The G4 runs Marshmallow while the G5 runs Nougat, and with the G5 now Quickpik is being a bunch of trouble. I asked about my new problem on the net and after some digging found an updated version of the Quickpik app tweaked for Nougat, now it and my G5 are playing nice with each other. So consider searching the net at large if you're determined to hold on to a troublesome app.
The moral of what I'm trying to say is that when you change OS, even an update, things can change within and suddenly apps that worked well may become a problem or not work at all in extreme cases. It's like upgrading your PC/Mac at home, OS files get moved around in the new OS and suddenly programs can't find files they're used to finding with the old OS and don't run without issues. Hopefully you backed up your data as at best you might just have to update some apps, but at worst you could look into finding new replacement apps that do the same things but aren't what you're using currently... Or look into possibly retro-grading and removing the patch(es) which might prove impossible and also could leave you vulnerable to whatever they patched.
Amazon also has its own Android app store called the Amazon Underground. They have many apps similar if not identical to the Play Store, but you'll have to side load any of these apps. But you could give them a try and they might be just different enough that you could download the same app as what's giving you trouble and it'll play nice with your updated OS.
Good luck!
