PDF file signed - IRM and label not applied

Hi,

We have some document libraries in sharepoint online protected with IRM. Recently I checked that some users have these libraries synced, so they don't see the error message that pdf signed are not allowed in a library protected with IRM. Then, pdf is synced with sharepoint online but anybody can open or download de file. 

I am not finding a solution to resolve this problem. Seems that the only option is disable IRM in these libraries, but this way I will lost this protection. Is there some kind of solution or alternative for this issue?

Thanks!