I think you answered your own question. If Tapatalk has been updated, it's likely the old version had bugs which have been fixed as well as cosmetic changes. I think it is very unlikely your phone has been hacked. You can actually get free antivirus software for android phones.

When did the "new version" come out? I am not that familiar with Tapatalk, stopped using it a long time ago.

I know you might prefer the old version, but if the new one fixes your problem. Then using it is the obvious answer.

*edit* - I have just seen that other people have posted that they have had strange problems with Tapatalk... so... it will just be bugs in the app. I've looked over a few of the patch notes for Tapatalk which do mention fixing bugs... it really is always a good idea to keep software up to date.