Amber Prison 
Artifact
You may choose not to untap this artifact during your untap step.
{4}, {T}: Tap target artifact, creature, or land. That permanent doesn't untap during its controller's untap step for as long as this artifact remains tapped.
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Cards that do something similar expected a card that isn't here?
Reports become test cases the matcher is graded against before an update ships. Delvefall isn't a chatbot guessing, it compares the printed rules text of every card with plain math, and examples like yours decide which differences should count.
Vedalken Certarch 90%
#26972
"{T}: Tap target artifact, creature, or land. Activate only if you control three or more artifacts."
Ashnod's Battle Gear 100%
$0.20#30335
"You may choose not to untap this artifact during your untap step."
Entrancing Lyre 100%
$0.11#22944
"You may choose not to untap this artifact during your untap step."
+1 more matching line
Vedalken Shackles 100%
$7.66#11447
"You may choose not to untap this artifact during your untap step."
Deserter's Quarters 100%
$0.05#29280
"You may choose not to untap this artifact during your untap step."
+1 more matching line
Flowstone Armor 100%
$0.14#31381
"You may choose not to untap this artifact during your untap step."
+1 more matching line
Helm of Possession 100%
$5.07#14749
"You may choose not to untap this artifact during your untap step."