On many stock portfolio trading systems, when backtesting daily data, different stocks have different start dates. For example, MSFT started much earlier than META.
It would help to know if the lookback period has been satisfied on a per-asset basis. It can be like this:
LookBack = AssetLookback = 100;
Then loop through the assets and check for (!is(ASSETLOOKBACK) && !is(LOOKBACK) && !Init) before trading on any given asset.