"iOS 13 apps are not managing their memory properly" would be a more accurate statement.
I am not sure that this is 100 % true. I understand that this might be true for the specific case of youtube. I have the same issues on my 2018 6th generation iPad with Safari and Books. Both applications are created and maintained by Apple.
Now either there is also iOS bug (and not just Devs not adapting their apps to iOS 13) or even Apple own Devs cannot adapt let alone others.
Either way I don't care. This is unacceptable. They need to fix not ASAP but for 2 months ago.