Fixed it. In my startup.m file, I set all LineWidth values to 1 in a for loop as follows:
lineWidth_list_factory = fieldnames(get(groot,'factory'));
index_lineWidth = find(contains(lineWidth_list_factory,'LineWidth'));
for i = 1:length(index_lineWidth)
default_name = strrep(lineWidth_list_factory{index_lineWidth(i)},'factory','default');
set(groot, default_name, 1);
end
Apparently a new property called defaultAxesGridLineWidthMode is added to the relevant list, and its value is either 'manual' or 'auto'. I fixed the issue with a simple if statement:
lineWidth_list_factory = fieldnames(get(groot,'factory'));
index_lineWidth = find(contains(lineWidth_list_factory,'LineWidth'));
for i = 1:length(index_lineWidth)
default_name = strrep(lineWidth_list_factory{index_lineWidth(i)},'factory','default');
if ~contains(default_name,'Mode')
set(groot, default_name, 1);
end
end
And the problem is fixed.