Thanks, please do report back with the findings, I checked the release notes and known issues and there is a similar known issue to be corrected in an upcoming release, but it is mentioned as being specific to MS390, doesn't mention MS355, but Support can investigate with the MS team. Do you happen to have any other models of APs that can connect at 2.5, or you only have MR53E APs? Just wondering if there's a way to gather any additional data points. If not, you could work with your Meraki rep to spin up a trial of an MR46 for example also running 26.7 to see if it happens there also and further prove it may be the MS doing this.
Also, if you go to Switch > Switch Ports and turn on the "Link" column in the flex table, and then in the search box type "lldp:MR53" and just look at those switch ports that have active MR53E APs connected, you can sort and reverse sort on that Link column and export the table. If you check back on that periodically, are the results different, do the ports tend to sometimes report 1G and other times 2.5G, or do they consistently start out at 2.5G when the AP boots, stay there for a perhaps random period of time, and then report 1G. Wondering if once they report 1G, if they stay that way or might report back at 2.5 at any point.
In addition, wondering if it really bounced back to 1G, or if it's actually connected still at 2.5G and it's some type of cosmetic UI bug. With that lldp:MR53 filter in place, make sure the Sent Bytes column is on, reverse sort on that to find the most heavily utilized MR53, and click into that ports details page. Check the port traffic over the last day or week, see if you might spot any spikes that were in excess of 1G even if/when the port was reporting 1G connectivity. That port traffic graph isn't super-granular or adjustable, might be difficult to spot. Support might be able to dig deeper into the details to better determine if it's a cosmetic error or if the port is definitely downshifting from 2.5 to 1Gbps. But on that note, when the port is reporting 1G, the AP itself is also reporting 1G on the AP's status page?
