Hi @SaffronGlint429,
The “spikes” have been noted by the developers.
Please see this post under “Feedback Snapshot”:
[BLOG] January 14th, 2021 Development Update - Community / News and Announcements - Microsoft Flight Simulator Forums
Update on “Spikes on Terrain” bug: Our team has successfully found the root issue. The fix is currently in testing. Thanks for your patience!