Continuing this line of posts.
If the period prior to Moshiach is akin to the period in Mitzriam regarding practice of Sod HaIbbur, then it is obviously not that during Galus Mitzraim that leap months were not intercalated.
There is hidden power within the 19 year cycle, the Ibbur Cycle, and this can only be used by Am Yisrael by the Sanhedrin's use of the leap cycle.
So what is / was missing is the knowledge of this sod and the actual usage thereof.
This, of course, is not the entirety of Sod HaIbbur, as can be seen from Rosh Hashanah 20b.