Play melody looping in the background slow down

Describe the bug
Arising from https://support.microbit.org/helpdesk/tickets/86759 (private)

When playing a melody looping in the background, some are slower on the second and subsequent plays.

• ode 16s then 22s,
• wedding 8s then 8.5s

Other built in melodies don't seem to be affected.

It sounds like perhaps repeated notes are longer?

To Reproduce
Steps to reproduce the behavior:

1. Go to https://makecode.microbit.org/_K818E6KF7eX1
2. Download to micro:bit V2
3. Open device serial console
4. Press button A
5. Wait for 2 plays
6. Hear longer notes on second play
7. See longer time in console
8. Press B to stop

Expected behavior
All plays are roughly the same length.

micro:bit version (please complete the following information):
V2.X

Desktop (please complete the following information):

• OS: [e.g. iOS] Windows
• Browser [e.g. chrome, safari] Chrome
• Version [e.g. 22]

Start by running the linked MakeCode program on a micro:bit V2 and use the serial console to compare the first and second melody durations. Trace the background melody playback path and reproduce the repeated-note slowdown for the affected melodies; done means consecutive plays have roughly equal lengths and pressing B still stops playback.