When I try to resize a (page-anchored) frame by dragging, the moment I release it, it jumps to the top of the page.
Try to perform your actions in Safe Mode on a new document and if that works, then reset your user profile. Otherwise provide a minimum of information, like operating system & its version, LibreOffice version, document format used, problem existing in new and/or existing documents, decent of documents (converted from .doc/.docx ?), problem new or existing since ever.
PS: I cannot confirm the behavior described in LibreOffiice 6.3.3.2 (openSUSE 15.1) using TDF packages.