Created attachment 30019 [details] Test code that demonstrates the bug If POI writes a formula that contains an ISBLANK() (or NOT(ISBLANK())) immediately within an AND/OR, Excel always evaluates it to false. However, if within Excel you rewrite the formula without modifying it (e.g. F2 then enter), then Excel will evaluate it correctly. This only occurs for HSSF, not XSSF.
I just noticed that IF(TRUE, ISBLANK(A2)) also has the same issue (returns false), whereas IF(ISBLANK(A2), TRUE) does not!