Conditional formatting for date column randomly stopped working

This applies to Sharepoint Online (Office 365), new experience lists.

I have some formatting set on a non-date column, which looks at a date column called "next deadline". This date column is automatically calculated based on other columns.

For about a year, this column formatting worked, and would display deadlines by colour, depending on how close they are to now (red, amber, green).

Randomly one day, I think it was about August of 2018, this stopped working, and now shows ALL deadlines in green.

Can anyone spot an error, or provide a solution?

Here's the code I wrote, which definitely worked for 6-12 months after I wrote it!

My apologies for the formatting of the code below, I tried but wasn't able to format it properly as code, with correct indentation.

{
"$schema": "https://developer.microsoft.com/json-schemas/sp/column-formatting.schema.json",
"debugMode": true,
"elmType": "div",
"txtContent": "@currentField",
"attributes": {
"class": {
"operator": "?",
"operands": [
{
"operator": "<=",
"operands": [
"[$Next_x0020_Deadline]",
1
]
},
"",
{
"operator": "?",
"operands": [
{
"operator": "<=",
"operands": [
"[$Next_x0020_Deadline]",
{
"operator": "+",
"operands": [
"@now",
86400000
]
}
]
},
"sp-field-severity--severeWarning",
{
"operator": "?",
"operands": [
{
"operator": "<=",
"operands": [
"[$Next_x0020_Deadline]",
{
"operator": "+",
"operands": [
"@now",
259200000
]
}
]
},
"sp-field-severity--warning",
"sp-field-severity--good"
]
}
]
}
]
}
}
}