Not sure those are included in your list.

$$A_t = B_t$$
$$A_t = B_t$$

displays only one:

$$A_t = B_t$$ $$A_t = B_t$$

abc
$$A_t = B_t$$
$$A_t = B_t$$

displays two:

abc
$$A_t = B_t$$
$$A_t = B_t$$

I try to render the equations:

• The first one with just an underscore _:

$\_$

$_$

• The second also with an underscore _:

$$ mean\_Day_{i, j} = \sum_{years} Day_{i, j, k} $$

$$ mean_Day_{i, j} = \sum_{years} Day_{i, j, k} $$

• The third with a sharp #:

$$ mean = \#group_i + \#group_j $$

$$ mean = #group_i + #group_j $$

I saw that you covered the subject well. Maybe I missed something or these are bugs in the formula display under Github.

Thank you for your opinion!