Description of the bug

Using lightning infill, I have just recently encountered two examples where lightning infill doesn't get anchored to the perimeter. When this happens, the lightning infill completely fails. I tried increasing both infill_anchor and infill_anchor_max to 1000 and this doesn't change the behaviour. Here's an image of what the infill looks like with those settings:

You can see from this image that the bottom portion of the infill is not anchored but the higher portion eventually becomes massively anchored. To see why this causes the infill to fail, we can zoom into a single layer of the unanchored infill:

What's also interesting is that the anchoring occurs as soon as a second bolt of lightning starts to form. It doesn't even have to be on the same perimeter (see the cube example, below):

With no way to ensure that the infill is anchored, I am forced to determine lightning infill is completely usable.

Project file & How to reproduce

To reproduce load this 3mf project into pretty much any version of PrusaSlicer (I can reproduce it back to 2.6.1):

BUG - legs.zip

and slice it. After slicing, examine the infill (for example, go to single layer view and look at the layer at z=55mm, this clearly shows the lack of anchor). Here are two made up and very simple and small examples that also demonstrate the problem:

lightning-infill-not-anchored.zip
lightning-infill-not-anchored-cube.zip

Checklist of files included above

• Project file
• Screenshot

Version of PrusaSlicer

2.9.3 (also 2.9.2, 2.9.0, 2.8.1, ..., 2.6.1)

Operating system

Windows 11

Printer model

Custom printer although it also reproduces on the built-in Mk 4 profile.