The lime and gypsum question is real and it is a different problem from yours, though worth understanding because it will come up.
Old solid walls have no damp proof course and are designed to breathe. Moisture gets in and evaporates back out. Lime plaster is porous and allows that; it also handles small movements because it is softer than the wall.
Gypsum and cement are much less permeable and much harder. Put them on a solid wall and two things happen over time: moisture that used to evaporate through the surface is trapped and goes somewhere else, and the hard patch does not move with the soft wall around it, so it debonds at the edges.
The classic failure looks like a patch that stays perfect in the middle and cracks in a neat rectangle around its border, sometimes with damp appearing next to it rather than in it.
That is not what you describe: same line, four times, no damp - so I would treat yours as movement first.
But the general rule is worth adopting anyway: on an old solid wall, match what is there. If the existing plaster is lime, patch with lime. Small filler repairs in gypsum are usually tolerated; large areas are where it bites.
How to tell what you have: lime is softer, scratches with a fingernail, and is often on timber laths rather than on the brick.