I don't see that the example cited is directly comparable to the present discussion.
When the decision was made to create two new album charts - Artist Albums and Compilations Albums - albums were assigned to one or the other chart, but continued to be listed together on the Comprehensive Albums Chart, a chart that is still compiled and which is the direct successor of the "old" album chart. New charts were created, but handling was consistent.
And similarly, when new rules are introduced, handling must remain consistent. When it became obvious that, not only were record companies willing to release formats that were not combinable in view of the rules in force at the time, and further, that the public was willing to buy them in large quantities, it led to a rethink in the chart rules.
The difference with Lady Gaga, who you singled out, was that the formats released by Interscope in the UK were designed to meet the requirements of the chart rules in force at the time. The reason that the "Fame Monster" EP was not released as a separate disc in the UK (as was the case in the US and other territories) was because this would have prevented sales of "The Fame Monster" being combined with "The Fame" - and there would then have been 3 separate albums tracked in the UK:
The Fame Monster (EP)
The Fame + The Fame Monster
By failing to make the EP available as a standalone format, they were able to combine The Fame Monster with the previously available The Fame.
The rules are published and well known to the record companies. If there's anything strange, it's that Universal knowingly released a format that was not combinable for sales purposes - not that the OCC has flipped and invalidated previouly compiled charts by now deeming the two formats combinable. Universal knew what they were doing - and they likely lost out on additional weeks at Number 1 for Back in Black by having sales split between the two competing formats when, if they had released a different configuration, all those sales would have counted for a single place.
Universal knew what they were doing, and if they were happy to do so knowing the rules at the time, they should not expect later rule changes to apply to their benefit.