I've seen this bug in one resource so far, Zacharias' "Biblical Greek Made Simple", but it may be existent in other resources that use tables as well.
The table I'm looking at as an example - the bug is manifest in all tables in the book I have tried - has the typical grid with thin lines (some fields merged horizontally, but not vertically as some others ideally should be - I'd suggest changing that if possible, but that's not the bug). Thicker horizontal lines are used to group portions of the table, such as the column heads and other portions (the difference between thin lines and thick lines is not strong enough for my eyes - again I'd suggest changing that, but that's not the bug either).
The bug is, that Logos changes randomly which horizontal lines are thick and which aren't, subject to my scrolling position, i.e. one notch of the mouse wheel up or down (or using the scroll bar, or even changing resource scale) will make the table structure look different. Difficult to explain but look at my screenshots:

The screenshots above were made with program scaling at 120% and content scaling at 110%, but the bug persists and occurs at both settings at 100% as well. The bug seems to exist for vertical thick lines as well, compare the optical structure of those two tables (now settings at 100%/100%):

When looking for whether this is resource specific or pervasive, I found that many tables in other Greek grammars - such as in Mounce's popular Basics of Biblical Greek - will not use a grid at all, or very minimal (such as to delineate row and column headers). In my quick survey I didn't look at them all, but I can't remember a resource which uses thin and thick lines to structure tables - it may well be that this is a long-standing bug (or design limitation) that just wasn't taken into account when producing this resource. EDIT: But I found at least one table in another resource where a grid of thin lines was used and the header was sett off by a thick line:

The thick line doesn't randomly move in this table, so it seems that it is possible to work this. If not so, /EDIT and if it is too costly to fix the bug, I'd suggest using color coding like in Huffman's "Handy Guide to New Testament Greek":
For the record: running Logos 8.12.0.0017 on Windows 10