Reporting a bug on someone's cards are fine, and encouraged to make sure cards work as intended. When reporting, however, guidelines need to be set and criteria must be known in posts to assure the maker knows what you're talking about.
Before you make a bug report, make sure you check to make sure it has not already been reported by another user(s).
For starters, I'll talk about the title/subject. It would be nice (although not required) to have the archetype's name in the title/subject in brackets (preferrably in the front). Also, do not be vague about the bug, even in the title. (The harder it is to know what the bug is, the harder it is to fix if necessary.)
Examples: [G-Vine] Cannot Fusion Summon Sopherena, [Blue Striker] Field Spell cannot be activated
Some questions to consider answering when making your bug report are:
What YGOPro Simulator and version am I using? (This is more or less required; different sims have different cores, even if slightly. An archetype made in KoshiPro might not work on Percy and vice versa.)
Do I have a replay?/Can I get a replay?
Is the bug consistent in certain scenarios? (A "bug" consistent vs AI but not a player is not really a bug.)
What are all of the cards or other variables (LP, Deck Size, etc) I think are relevant to the bug?
Because the custom cards follow OCG rulings (if a precedent is available; otherwise it's the card maker's choice on the rule), you can report that and send an official OCG source stating how the scenario should be.
Example: I make a card "Lumina the not-Lightsworn" and its effect is: "Once per turn: You can discard 1 card, then target 1 Level 4 or lower "not-Lightsworn" monster in your GY; Special Summon that target." but I script it so that I can use the effect without a proper target so long as I discard a card during the cost to be able to target. Since this contradicts the ruling for Lumina, Lightsworn Summoner, you can respond like this:
The effect of Lumina the not-Lightsworn contradicts the current OCG ruling for Lumina, Lightsworn Summoner, which states that you cannot activate Lumina, Lightsworn Summoner if you have no targets.自分の墓地にレベル4以下の「ライトロード」と名のついたモンスターが存在しない場合には、『そのモンスターを特殊召喚する』モンスター効果を発動する事自体ができません。
Source: https://www.db.yugioh-card.com/yugiohdb/faq_search.action?ope=4&cid=7594&request_locale=ja
The effect should be changed to match OCG's ruling.
