Okay, caught the boot output on UART, the whole thing is here: https://pastebin.com/BGtLGEkL
However I think the relevant section is this:
[ 2.999872] mmc0: new ultra high speed SDR50 SDIO card at address 0001
[ 3.014550] udevd[1189]: starting eudev-3.2.11
[ 3.017714] mmc2: new HS200 MMC card at address 0001
[ 3.020107] mmcblk2: mmc2:0001 SLD64G 57.6 GiB
[ 3.030252] mmc1: card 5048 removed
[ 3.032259] udevd[1191]: inotify_add_watch(5, /dev/mmcblk1, 10) failed: No such file or directory
[ 3.038169] udevd[1208]: inotify_add_watch(5, /dev/mmcblk1p1, 10) failed: No such file or directory
[ 3.052389] mmcblk2boot0: mmc2:0001 SLD64G 4.00 MiB
[ 3.054654] mmcblk2boot1: mmc2:0001 SLD64G 4.00 MiB
[ 3.058597] mmcblk2rpmb: mmc2:0001 SLD64G 4.00 MiB, chardev (242:0)
It seems like one of the storages gets remapped to mmc2, and mmc1 gets dropped? But I have no idea why.