"There is a long but safe bridge nearby."
Translation:Lähellä on pitkä mutta turvallinen silta.
pitkä agrees with object silta
"Silta" is the head of the subject, not the object.
why does turvallista appear as or more likely to agree than turvallinen?
I'm not sure what you mean because the nominative singular form "turvallinen" is the only option that could conceivably make sense since the head of the noun phrase is in the nominative singular form "silta".