I'm pretty sure 'Slide Over to Make Room' has only ever applied to clips, and not tempo/meter changes or markers. In any case, that function can be problematic even for clips (as noted in the SONAR Readme for quite a few years), and I recommend you use Project > Insert Time/Measures to make space for the paste in advance, and then paste in 'Blend' mode.
Also, you need to ensure that there is a number of measures between the Insert point and the next meter change greater-than-or-equal-to the number of measures you're inserting, or you will see extra measures inserted; this is a bug. If you need to insert more measures than will fit before the next tempo change, you'll have to Insert in stages - i.e. insert 2 measures, then 4, then 8, etc, depending on how may you need.
Unfortunately, you're hitting one of SONAR's long-standing soft spots; I believe all these shortcomings/issues persist in the current release.