Have you got in touch with Revolut’s support? In addition you could also message @AndreasK, maybe he could help out, though he isnt likely to reply before Monday.
At this point it is impossible to say if it is related to the older version you are running or to a general issue that would occur also with a current version.
As for your phone, which one is it? Have you checked if there is maybe an Android update available for it? If you are going to get a new phone, try to get one with at least Android 7 - Android 9 is already around the corner and will be released in a while.