Fastbooting a SailfishOS recovery image is not supported on the Xperia 10 II (only).
I.e., trying to fastboot the recovery image on a Xperia 10 II is futile!
For the official workaround, see [Jolla’s Zendesk] Xperia 10 II: How to use the Recovery Mode.
I.e., currently you have to overwrite the two boot partitions with the recovery image, by flashing it there. After performing your work with the recovery image, you have to flash the boot image to the two boot partitions A and B.
Technical background: This is well known (since Jolla designed how to use Sony’s partition layout on the Xperia 10 II), a proper solution is implemented by Jolla, but not yet released; click here for the details and Jolla’s plans for this.