spacebar key not typing space with external keyboard

With an external keyboard, the spacebar does not type space.

space -> no space.
alt-space -> gives space.
shift-space -> gives space.

iSH 1.2.2 (build 178)
iPhone 7, iOS 15.1
Same behavior for all keyboards tried:
Atreus USB keyboard, SUNTRSI USB Camera Adapter
Apple A1242 USB keyboard, SUNTRSI USB Camera Adapter
Apple A1644 Bluetooth keyboard

The spacebar does work as expected in other apps, like Notes and Messages.

Start by reproducing the issue with an external keyboard on iOS 15.1, comparing plain spacebar input with alt-space and shift-space. Trace the external keyboard input path and verify that an unmodified spacebar inserts a space while the reported modifier behavior remains correct.

Written by the indexing model from the issue text.