The wifi standard has specified that clients decide to switch to another access point in Mesh setups. The router has no task here.
Fortunately some mesh systems have now implemented AP steering, a mechanism that is built in the router and force clients to move.
I know my Fritz mesh uses AP steering, not sure about synology.
Sometimes new drivers on the client help.
I have the same issue. I know that devices that do not support 802.11v have issues with roaming. What is strange in my case is that it is one single Alexa device that decides to connect to the access point with poor connection. I have different Alexa devices nearby that connect to the closer access point. I pretty much use standard settings. Does anybody have an idea how to optimize this?