orthography - How are Japanese words spelt in English?

When they are writing material in English, I sometimes see native speakers of Japanese misspell English words that were derived from Japanese.

For example, I've seen "tunami" written instead of "tsunami", and "ninjya" instead of "ninja". I've also seen "geisya" instead of "geisha", and "Kyusyu" instead of "Kyushu".

My suspicion is that they know how these words are spelt and pronounced in Japanese, and that they convert it into the latin alphabet in a way that is different to how the word is spelt in English.

For example, with "tsunami", they've taken the first syllable, つ, and rather than converting it to "tsu", they converted it to "tu". Possibly because in certain ways of writing Japanese using the latin alphabet (romaji), the way that つ would be written is "tu".

Are there general rules about how English words that are derived from Japanese words are spelt? For example, are they generally spelt in a way similar to Hepburn romanization, except often without the macrons?

Even if the rules don't apply 100% of the time, rules that apply 90% of the time, with just a few exceptions may be preferable to some than memorizing all such words by rote.

An exception here is for wasei-eigo words such as "salaryman". When converting made up English into English, the English words that were used in the wasei-eigo are used ("salary" + "man", rather than "sararyman")

Any possible answers that a person who is not a linguist can easily understand?

Meta: I previously asked this on English Language Learners, but it was closed there (currently with one re-open vote), and in this comment on Japanese Language & Usage (not my fault!), it was suggested I ask on English Language & Usage.