FYI:
On an iPhone XR, fully updated, no betas.
Markdown document, edit mode
BUI pulldown, select strikethrough, doesn’t do anything. Selected text or just a cursor, no result
On iPad, also fully updated, no betas
Markdown document, edit mode
Selected text or at cursor, tapping strikethrough symbol inserts proper text.
All other formatting selections seem to work properly on both iOS and iPadOS.