Inconsistent or unpredictable behavior of the "offset along line" in marker/hash symbol layer

[DelazJ]

What is the bug or the crash?

Among marker/hash line symbol layer properties, there's the "offset along line" allowing to "adjust" the marker/hash position with a custom distance. It can take positive or negative values. I'm trying and failing to understand the expected behavior in either case and whether these are coherent.
I'm using the "with an interval" placement option on an not closed line feature.
1/ If I set a positive offset, "original" markers are offset along the line until the end of the line, and they disappear if they would not be placed on the line. that means number of offset markers is always =< number of "original" markers
2/ If I set a negative offset value, then I get a negative offset for the first vertex of the line, meaning that there's a marker that IS NOT on the line feature (first inconsistency)
2a/ If abs(offset) =< interval, then I always get same number of offset and original markers
2b/ If for some reason, abs(offset) > interval, then more markers are added out of the original line. And here I really fail to find what the new markers belong to, from where they are offset

Using first or last vertex placement modes, which are the other modes the "offset along line" option is currently compatible with (see #48980), this does not occur: whatever positive value and unit you use the markers are ON the line or removed. They just do not react on negative values (I don't know what to think of this behavior, yet)

Steps to reproduce the issue

alongLineNeg.mp4

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Additional context

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[nyalldawson]

Using first or last vertex placement modes, which are the other modes the "offset along line" option is currently compatible with (see #48980), this does not occur: whatever positive value and unit you use the markers are ON the line or removed. They just do not react on negative values

For reference the behaviour depends on the geometry -- if it's a closed ring then the marker placement will "wrap around" the ring.