I've now been using the R5 with a variety of lenses for several weeks. Running firmware 1.1 I've noticed the following:
1) On occasion the camera won't wake up when it's been sleeping with power on. The only way to revive it has been to remove and reinsert the (full) battery.
2) The camera will occasionall spontaneously enter video record mode. This happens when walking along with it dangling on my camera strap. I'm quite sure it's not inadvertent pressing of the red movie record button. This one may be user error but even after studying the manual I don't know how this can happen. I do think it's a camera firmware bug.
3) This is probably a problem with the new Canon RF 100-500mm f/4.5-7.1L IS USM lens firmware. Occasionally, when the camera has been sleeping with the lens attached, when you pick up the camera, wake it up, and attempt to use the lens, the lens keeps it's iris wide open. Auto iris does not function. The way I've been able to recover is to turn the camera off and back on again. The way around the problem has been to turn the camera off when I set it aside with that lens then turn it on when I pick it up to use it again.
I hasten to add that I am mostly very pleased with the R5 and its lenses. I believe the autofocus is the best in the business and the image quality is utterly stellar. The criticism about the possible duration of video recording is not relevant to me. For video I use a video camera (c.f. the great Canon C300 III)
I know this was 1 1/2 yrs ago, but did you ever find out what the cause was? Just bought the R5, my 100-500 finally arrived yesterday and I went out this morning to try it out. Three or four out of ten times +/- I had to do the same thing. On top of that, a couple of times it waited between 5-10 sec and then turned on. I can hear the lens AF "waking" but nothing from the camera. Very frustrating! Thanks!
I have an issue with my R5 (latest firmware)
Over the past two mornings the LCD screen has frozen when tracking a Squirrel 5 metres in front of me.
I was using my 300mm f2.8 MKII with basic adapter the first morning, servo tracking mode , focusing on subject no pictures taken ( I used BBF) and suddenly screen freezes with exactly the same view as before, looks like viewing an image on screen but with the blue focus points . This lasted about 6 seconds then camera makes a soft clunk like powering on and all is normal. Same this morning , happened twice in 3 hours with RF 70-200mm lens .
Hope Canon are reading this, great camera, best they have ever made 🙂
Someone else on https://www.dpreview.com/forums/post/64347077 has similar problem
"Same thing happened to me on the R5 and 100- 400 II lens. The camera just froze
Another freeeze problem here https://www.dpreview.com/forums/post/64220495
"I had 2 instances today using animal eye focus on a bird on a wire. I was waiting for it to fly or move and the camera froze during identification of the eye, shutter could not be released. I had to push the menu button and start over.
It froze within a minute of keeping the camera focused on the bird.
I’ll continue to test this. Maybe I’m missing something,"