'Start of Line' anchor bugs out and places labels at end of line instead

[anitagraser]

What is the bug or the crash?

Using 'Start of Line' label anchor does not result in reliable labels. Indeed, in some cases (this changes by zoom level), the label is placed on the end of the line instead of at the start.

In the following example, the problem is that it wrongly puts the Campus label there on some (but not all) zoom levels:

label-anchor-bug.mp4

(HT to @pigreco for reproducing the issue and recording the screencast)

Steps to reproduce the issue

1. Load the the line layer: https://nx60860.your-storageshare.de/s/KSdKEWknNpQQjgf
2. Apply the style: https://nx60860.your-storageshare.de/s/GywQXwMXyTFBGzP
3. Zoom to the area shown in the video above
4. Observe how changing the zoom level leads to different (sometimes wrong) labeling results

Versions

QGIS version	3.40.0-Bratislava
QGIS code branch	Release 3.40
Libraries
Qt version	5.15.15
Python version	3.12.7
GDAL/OGR version	3.9.2
PROJ version	9.5.0
EPSG Registry database version	v11.016 (2024-08-31)
GEOS version	3.13.0-CAPI-1.19.0
SQLite version	3.47.0 (Compiled) 3.47.2 (Running)
PDAL version	2.8.0
PostgreSQL client version	unknown
SpatiaLite version	5.1.0
QWT version	6.3.0
QScintilla2 version	2.14.1
OS version	Linux Mint 22
Active Python plugins
MemoryLayerSaver	5.0.2
processing_trajectory	2.3
plugin_reloader	0.17
grassprovider	2.12.99
MetaSearch	0.3.6
db_manager	0.1.20
processing	2.12.99

Supported QGIS version

• I'm running a supported QGIS version according to the roadmap.

New profile

• I tried with a new QGIS profile

Additional context

For comparison, if Geometry Generator with start_point( @geometry ) is chosen instead, the labels are placed correctly.