My theory: It is Vodacon.
I have been a long term prepaid Cell C customer from 2007 on, never had problems with roaming on VC in the past. But mostly made use of the own Cell C tower nearby. Also been a long term data customer, their Giga 200 packages kept me going since 2013 somewhere.
Unfortunately cell C switched off the local tower late 2019. But roamng on MTN always showed up as Cell C and altough data speeds were very slow for some months but great again the last 6 weeks or so, never had this roaming issue.
So Telkom Mobile recently installed a new tower in this area, and it showed on my Dual SIM phone as that TM was NOT roaming, no R. So I bought a Telkom only prepaid data bundle, to test it out, which I never got to work. So apparently that tower is not working yet. After that my TM SIM went on/off, later completely off for almost 14 days. Whatever I tried, manual network search, Telkom SA roaming 2G, 3G or 4G forbidden, it failed. The SIM was just dead for almost two weeks, then suddenly came back to live, and works again.
Now VC is doing the same with Cell C customers roaming on their network. I never trust them. Might all be intentional.
Edit, addition : Agree, this reigning duopoly appears again to act like a cartel, trying to put the final nails in the coffin of the two smaller ones. Both of them, TM and CC did not do themselves a big favour, by making all kind of mistakes themselves.
At least their excellent prepaid data bundle 50Gb all time + 50GB nite, valid for 180 days, works still very well for me.
I never had issues up until this 'roaming on Vodacom' was pushed out.
MTN was great for us, but now, since being forced to roam on Vodacom all our phones are having reception issues.
I have tried to manually select 'Cell C' and 'MTN' both say 'Forbidden'
The MTN 'roaming' worked flawlessly.
I guess I will be leaving Cell C for MTN now.