I'm not 100% sure but I think I found a bug related to PAL and sprite fill patterns. The manual says

PAL() resets to system defaults (including transparency values and fill pattern)

However, the following cart shows that PAL does not clear out the fill pattern applied to sprites.

fill_pattern_is_not_reset

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fill_pattern_is_not_reset

by andrewedstrom

Cart #fill_pattern_is_not_reset-0 | 2021-03-23 | Code ▽ | Embed ▽ | License: CC4-BY-NC-SA

Here's the code for that cart. Note that you have to uncomment fillp() in order to reset the fill pattern.

function _draw()
    cls()
    pal()
    --fillp()
    sspr(8,0,8,8,10,10,24,24)
    sspr(16,0,8,8,10,40,24,24)
    fillp(0b1111111111111111.01)
    sspr(8,0,8,8,70,10,24,24)
    sspr(16,0,8,8,70,40,24,24)
end

Please let me know if I'm misunderstanding something! It could be that this is expected behavior and I just misread the docs.