[2.0.55] Flushing pipe network doesn't flush non-fuel in fluid fuel slot but does appear to

[sparr]

I encountered this bug playing Pyanodon's mods, but based on discussion there I don't think this is a bug in the mod. It may be impossible to trigger this bug in vanilla Factorio due to a lack of vanilla entities with the necessary combination of features, but supposedly the features themselves are part of vanilla.

content below copied from https://github.com/pyanodon/pybugreports/issues/1011

What is the problem?

After attempting to flush it, molten glass stays stuck in the Glassworks fuel slot.

Steps to reproduce

1. Build a glassworks
2. Attach a pipe of molten glass to the fuel in/out port on the glassworks
3. Observe 200 molten glass in the glassworks fuel slot
4. Detach the pipe of molten glass
5. Observe 200 molten glass still in the glassworks fuel slot
6. Attach an empty pipe to the fuel in/out port
7. Observe 200 flushable molten glass in the gui for that pipe
8. Flush that pipe
9. Observe no flushable fluid in the pipe
10. Observe 200 molten glass still in the glassworks fuel slot
11. Remove and re-place empty pipe
12. Observe no flushable fluid in the pipe

The attached save is at step 5, the fuel slot has been primed to full with 200 molten glass. If you attach a regular pipe to the side of the glassworks, the glassworks fuel slot will stay at 200 molten glass, and the pipeline contents in the pipe gui will say 200 molten glass, and mysteriously 66.6 more molten glass will have magically appeared in the pipe. This seems like a bug, probably related to the bug I'm reporting but not exactly the same. Proceed from step 8, flushing the pipeline contents from the pipe gui, to reproduce the rest of this bug report.

[protocol_1903]

I got a similar report for PyBlock a while back about the fish farms having seemingly endless hidden fluidbox storage. Further search resulted in the discovery of a hidden output fluidbox that had a large capacity but was completely invisible, and separate from the fluid shown in the entity's recipe window.

[Rseding91]

Thanks for the report. This is now fixed for the next release.