@tomas209cawe had the same with our MX15.17/18 images when 15.20 came out, I believe as it fixes a significant performance bug. Interestingly it didn't propose to upgrade 15.14 devices. We also had similar with switches on MS11.25 as again there was what appears to be a significant bug fix.
As @PhilipDAth said the auto beta off feature is just so that a stable image is not suggested to go to beta, or automatically upgraded by Meraki.
We are using MX15.14 on the MX250 HA concentrators at our SD-WAN core, MX15.20 at most other sites that have HA pairs of MX100 or MX84, with a couple of the MX84 pairs on MX15.21. Our single internet access only MXs are all on MX15.21 and were on MX15.20 when it came out. We haven't, touch wood, had any issues...
If my answer solves your problem please click Accept as Solution so others can benefit from it.