The battery used in your camera is made up of several cells each of which supplies a fraction of the total battery voltage and joined in series they provide the total voltage needed. If one of those cells develops a defect your battery quickly goes from normal to nearly unusable because the entire effective battery storage (and current capability) is limited to that available from the weakest cell. This is a common form of battery failure where one cell develops a critical defect and that is probably what happened with your camera battery.
In the bad old Nickel Cadmium days the weakest cell would often be reverse charged by the stronger cells reversing polarity which was the major reason you didn't want to discharge the nicad battery too far. Modern batteries are more robust against this issue and also avoid the "memory effect" that occurred with certain rechargeable batteries that were frequently recharged before they were fully discharged and then would sometimes no longer allow a full discharge before going "flat".
In the distant past there was a clear distinction between a "battery" made up of several cells versus the common AA cell, D cell etc. which we now generically call batteries even though they are a single cell instead of a "battery" of cells.
I doubt if your battery was switched. If someone was going to take your battery the lowlife would likely have taken yours and kept his current nearly bad one.
Rechargeable batteries of all types have life limits which vary with the type of service and often we have used our batteries more than we appreciate; it was probably just time for you to get a new battery and I would stick with a Canon replacement. Not all third party batteries are of lesser quality but a lot of them are so it is a gamble I wouldn't take.
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
