When a provider specifies RTP port ranges, that is almost always the ranges that they will be signalling to you where to send the media to them. The rpt.conf port ranges are where Asterisk is specifying to receive the streams. Unless your provider is requring some really invasive rules on you, unlikely, then there should be no reason to change your ranges as they should send the media to where ever you tell them to and as such you shouldn't have to change the rtp.conf default configuration. The provider never dicates where it's going to send to, only where you are to send to it.
In either case, if you choose to change your ports, it's usually a good idea to have a range that is a lot more then adequate to cover the number of simultaneous calls you'll have (x2 since it's only even port numbers it will use).
It does appear form what you've described there may be a bug in the changing and resetting of default rtp port settings in Asterisk, so if so, it's a valid bug to report for us to explore. But, my recommendation would be to stick with the defaults if the only reason you made these changes was thinking you had to based on your provider. (Unless I'm wrong, and they really are dictating the 'from ports' for your RTP as if they would reject any traffic that didn't come 'from' their specified range, possible, but unlikely.)