Disabling the portal solves the spikes. Even new portals do that.

My config settings:

{
// When enabled, sends a message to players around the portal that the nether is spreading and that you can stop the spread with 'preventSpreadBlockAmountNeeded' of the 'preventSpreadBlockString' block.
"sendMessageOnPortalCreation": true,
// The message sent on portal creation.
"messageOnPortalCreation": "You feel a corrupted energy coming from the portal. The nether will slowly spread into the overworld unless %preventSpreadBlockAmountNeeded% %preventSpreadBlockString% are placed within a %portalSpreadRadius% block radius around the portal.",
// When enabled, sends a message to players around the portal that the nether spread has stopped when the portal detects new 'preventSpreadBlockString' blocks.
"sendMessageOnPreventSpreadBlocksFound": true,
// The message sent on preventspread blocks found.
"messageOnPreventSpreadBlocksFound": "With enough %preventSpreadBlockString% placed, you feel the corrupted energy fade.",
// When enabled, sends a message to players around the portal when it is broken.
"sendMessageOnPortalBroken": true,
// The message sent when a portal is broken.
"messageOnPortalBroken": "With the nether portal broken, the corrupted energy is no longer able to enter the overworld.",
// When enabled, shows the portal coordinates in portal messages.
"prefixPortalCoordsInMessage": true,
// The radius around the portal to which the nether blocks can spread.
// min: 1, max: 100
"portalSpreadRadius": 100,
// The delay in ticks in between the spread around the nether portal. 20 ticks = 1 second.
// min: 1, max: 72000
"spreadDelayTicks": 60,
// The amount of blocks that are instantly converted to a nether block around a portal when it is detected. If there are existing nether blocks within the radius, their count is substracted from this number.
// min: 0, max: 1000
"instantConvertAmount": 30,
// When enabled, blocks the spread effect when there are n (defined) prevent-spread-blocks (defined) within the radius.
"preventSpreadWithBlock": true,
// The amount of prevent-spread-blocks (defined) needed within the radius of the nether portal to prevent spread.
// min: 1, max: 100
"preventSpreadBlockAmountNeeded": 4,
// The block which prevents the nether portal from spreading. By default a coal block (minecraft:coal_block is the namespace ID).
"preventSpreadBlockString": "minecraft:beacon"
}