So I've also had this problem with my R5 and now my R3. Turn on the camera and nothing. If I remove and reinsert the battery? It turns on. When I got the R3 I updated the firmware and initially that seemed to fix it but I was shooting an event last night and it started this again. Turn on. Pull battery. Reinsert battery. I have tested this with and without the EF-R adapter and with several lenses (RF 24-105, EF 70-200, Sigma 24-70) and every situation I have the same issue.
I've seen that this was an issue and wondered if anyone has figured out what causes this? Thanks!