Although we use LR No and IMO No interchangeably, it not as simple as that, though they are a common numerical sequence. In 1987 the IMO adopted the already-existing LR numbering system, which had been in the current 7-digit form since 1969. I cannt remember the details of the various corporate changes involving LR, but the part of LR dealing with the IMO contract had morphed into a JV with Fairplay, there was a management buyout, and eventually they sold to IHS.
As I understand it, that structure did not not acquire all the historic data from LR, which continued to publish the Registers in their own name, but the IMO contract did involve acquiring the ship histories and other data for all those ships which were believed to be still extant and thus were acquiring formal IMO numbers. For the IMO contract or for wider commercial purposes they only required data on active vessels and I suppose that is what they got. So it is not surprising to me that IHS data does not contain ships which had already been deleted in the 1970s.
I suspect that IHS are just being a bit loose in describing the number as "invalid", when they are really saying that there has never been a ship with that IMO number, which is correct. Anyway, I will ask LR, though I not sure how quickly they can answer at the moment, not only due to COVID but also because they were already in the middle of a long rebuilding project at LR and a lot of historical material is in storage. Let's see.