Confirmed on Windows 10 Ent 64-bit en-US with recent master
Version: 126.96.36.199.alpha0+ (x86)
Build ID: 489eef894e7034873ad262f9dfca554022db1b09
CPU threads: 8; OS: Windows 10.0; UI render: GL; VCL: win;
TinderBox: Win-x86@42, Branch:master, Time: 2019-10-01_23:43:38
Locale: en-US (en_US); UI-Language: en-US
It is not limited to anchoring 'as character'; same behavior with default
anchor for frame 'as paragraph' or the other 'to character', 'to page' anchors
behave the same.
Resizing the frame--either with the Frame... dialog's 'Height' spinner, or
using its "grips" to drag--will reduce its size from the bottom up, likewise
enlarging with grips will add space to the bottom.
So seems size is always relative to top left corner.
But, a potential enhancement? Should the frame resize have an variable anchor
point like rotate has a pivot? But strictly internal, ODF should not be an
issue--the resize would be complete before writing out the document.