Coming from https://www.qcad.org/rsforum/viewtopic.php?f=76&t=7148 ...

I create help lines of infinite length inside a block, layer is "helplines", red, 0,25mm, DOT(2x). Inside the block I can see the lines at any zoom level. Leaving the block editor, the lines are getting shorter when zooming closer. Zooming far away the lines look like infinite length, zooming closer, the lines look like cut shorter, and zooming very close, the lines disappear completely.

As Husky confirmed:

Infinitive lines in blocks are displayed shortened in Model_Space. I would consider that a bug which pertains the QCAD Pro version too.