Device dehydration: "Reset recovery" with dehydrated devices enabled prompts for security key just after setting it

[richvdh]

STR:

1. Enable MSC3814 on synapse (experimental_features: msc2697_enabled: false msc3814_enabled: true)
2. Enable MSC3814 flag in well-known (extra_well_known_client_content: { "org.matrix.msc3814": true })
3. Log in. Observe:
   
4. Click "Verify with Security Key or Phrase".
5. Click "Reset all"
6. Click "Reset" to confirm. Observe:
   
7. Either generate a key or enter a phrase
8. Observe:
   
9. Click "continue"
10. Enter account password

Expected

Secrets are correctly reset

Actual

It's prompting me for the security key which I have literally just set.

Notes

This only happens if a dehydrated device previously existed.

If you enter the security key, then everything is reset correctly, but then you end up back at the start: #29120

You can also hit this flow from within the settings panel, by clicking this Reset button, though note that said button is being removed as part of #26468:

[richvdh]

It's prompting me for the security key which I have literally just set.

Actually no, it is prompting me for the old security key.