I have a test setup with 3 RB953GS-5HnT boards equipped with S-35LC20D and S-53LC20D single strand SFP Modules. I have an issue on reboots where the Routerboard will not initiate the SFP. It shows the Module as present, but does not activate the output or show any of the diagnostic information in winbox (see picture).
Re-seating the sfp brings it back to life, but as these will need to be enclosed at remote locations, reseating after each reboot is not an option.
Please try to upgrade your device to latest version of RouterOS - 6.27.
Between version 6.20 and 6.27 there are plenty of changes which might already fix your issue.
I applied the latest release candidate yesterday. With a Fiber Power level meter, I have noticed that the SFP’s will not output any power if they are not recognized at boot up, but the power is there instantaneously if they are. Oddly enough the boards with Routerboot 3.17 seem to be more stable than the boards with Routerboot 3.20. If I do a reboot, the sfp’s are still working and recognized by the board, but if i power them off and then back on then they are not initialized.
