The setting to add a HTML format to a textbox question will make the question disregard validation on iOS. Everything seemed fine to me in desktop interactions, however.
The recommendation for the time being is to simply not use this settings and instead use validation by itself.
We decided to not make any changes to this function at this point. We may consider making changes at some point, and we will keep you posted.
If you continue to run into issues with this feature, please let us know!