I have been hit by exactly the same issue, on a clean and up-to-date Win10 PC that has worked flawlessly for months.
I recently installed the excellent Affinity Photo and Designer applications. Today, when opening Designer, all text disappeared from most windows, exactly as you show in your video.
This strange behaviour is not a fault of Affinity nor your fault (even if Fritz_H insinuates so in his useless comment, that is not worthy of any decent community). However, it is clear from our identical experience that some components used by Affinity software trigger the issue on our Windows 10 installations.
I was lucky enough to try something that solved the issue for me (being a computer engineer might have helped, for once!). I post it here in case it helps you and future users affected by the same problem. Affinity software is too good to have its reputation spoiled by a rare Win10 conflict!
What solved the issue for me
The disappearing text seems to be caused by Windows 10's font cache. Resetting the font cache did the trick. Here's how:
Stop both the "Windows Font Cache Service" and "Windows Presentation Foundation Font Cache 126.96.36.199" services (e.g. by running "services.msc")
For me it was also necessary to disable "Windows Font Cache Service" to prevent it from immediately restarting once stopped
Delete all the contents of "%WinDir%\ServiceProfiles\LocalService\AppData\Local\FontCache"
Delete "%WinDir%\System32\FNTCACHE.DAT" (if it exists)
Optionally set the a.m. services in manual mode if you had to disable them at point 2
Optionally restart Windows
I wrote a short batch file that automatically does all of the above, I keep it on my desktop in case the issue happens again. I attach it here for your convenience, you can use it at your own risk (same goes for the instructions above). I hope it solves the problem for you as it did for me.
Fix font cache.bat