chess - Is it possible to collect pv moves ending with checkmate, using triangular tables?

I'm trying to figure out how a traditional chess engine (no AI) works, and now I'm trying to collect pv (principal variation) moves using triangular table. So I'm using a NxN table and a very simple implementation

private Integer alphaBeta(final int depth, int alpha, int beta) {
    ...
    pvLength[ply] = ply;
    ...
    while (move != null) {
        ...
        if (value > alpha) {  // (1)
            dPvTable[ply][ply] = move;
            nextPly = ply + 1;
            for (int p = nextPly; p < pvLength[nextPly]; ++p) {
                pvTable[ply][p] = pvTable[nextPly][p];
            }
            pvLength[ply] = pvLength[nextPly];
            ...
        }
        ...
    }
    ...
}

I am comparing the extracted pv with the pv resulting from the transposition table.

It works well if the pv doesn't end with checkmate. If it does end with a checkmate, the triangular table returns only the first (correct) move.

I have tried many changes but without success. The only thing that works (at the expense of search speed) is to change the condition (1) to

if (value >= alpha) {

But clearly this is not an acceptable change, because this way the search explores the tree up to the last move, and does not end at the first checkmate found.

I just wanted to know, if anyone had the same problem: is this behavior due to a flaw in my implementation? Or this method just can't return all PV moves when there are checkmates?