The iOS Joplin app currently adds 4 spaces on the left of lines when requested to increase their indentation level.
I would suggest using the same implementation as the one used by the Windows app, which is a tabulation.
I see there are discussions about which implementation is better, about plugins allowing to move from spaces to tabulations and vice-versa... but IMO at a given time and without any plugin things should be consistent in any case !
I'm submitting a pull request to fix this! Edit:Pull request.
I think the original rationale for using space indentation on iOS/Android was that mobile keyboards often don't have a tab key (and the legacy iOS editor didn't auto-indent). Defaulting to space indentation then would keep the editor consistent with manually-indented notes.